The present invention relates to an optical fiber cable constituted by fabricating a coated optical fiber into a cable.
In recent years, with the increase in demands for large capacity data transmission, an optical fiber cable that allows large capacity communication has expanded its fields of application to be utilized as a backbone line of a long haul communication network. At the same time, with, for example, the remarkable increase in utilization of communication lines by individuals represented by the Internet, construction of optical cable networks is in urgent need more than ever.
Under such circumstances, optical fiber cables are used under various environments, and thus development of a cable excellent in heat resistance and waterproof property is becoming important. In general, it is conceivably possible that, in the case of the optical fiber cables for underground installations, a sheath of a cable is damaged to cause water to penetrate inside the cable, and optical fibers in the cable are left being flooded for a long period of time. Thus, in order to prevent the penetration of water into the cable and migration of water in a longitudinal direction, various water blocking means are known and measures therefor are applied.
In addition, characteristics of an optical fiber cable depend largely on characteristics of its covering material and the like, not to mention characteristics of its coated optical fibers. For example, a transmission loss increases due to microbending that is given to a coated optical fiber because of a dimensional change of a covering layer.
FIG. 3 shows an example of a waterproof optical fiber cable that has been proposed recently. The optical fiber cable 31 has a cable core formed by covering an external circumference of a plurality of optical fiber ribbon 32 with a cushioning member 37 formed by twisting around a string-like fiber 33 having water absorbency in one direction, tension members 34 disposed outside the cable core, and a sheath 35 collectively covering the cable core and the tension member and which sheath is composed of a thermoplastic resin or the like.
In this conventional optical fiber cable, the cushioning member 37 having water absorbency is employed as a water blocking measure and plays its role.
In addition, in the optical fiber cable of this type, it is a general practice to use the string-like fiber 33 that is a super water-absorbent polymer, such as a polyacrylic acid-based polymer and a polyvinyl alcohol-based polymer, as the cushioning member 37 having water absorbency. When the string-like fiber 33 is used as the cushioning member 37, it is a general practice to wind the string-like fiber 33 around the coated optical fiber in a spiral manner in a fixed direction or in a reversed direction.
In such a conventional technique, it is necessary to pay attention to water absorption and swelling of a cushioning member. That is, it is necessary to select a super water-absorbent polymer to form a string-like fiber, and to form the cushioning member in a density as low as possible, in order to facilitate the fiber to absorb water to swell. However, it has been found that, when the fiber to form the cushioning member is used at a low density, its water absorption and swelling characteristics are satisfactory, but large shrinkage by heat occurs, and the string-like fiber shrinks due to high temperature hysteresis after fabrication of a cable. Accordingly, in particular, if the string-like fiber is wound in a fixed direction or in a reversed direction, there is a possibility that the string-like fiber excessively tighten a coated optical fiber to increase a transmission loss.
The present invention is an optical fiber cable that comprises: a cable core formed by covering an external circumference of a coated optical fiber or a collective coated optical fiber with a cushioning member; a tension member; and a thermoplastic resin sheath for collectively covering the cable core and the tension member, wherein the cushioning member comprises a blended fiber composed of a non-water absorbent fiber and a water absorbent fiber.
Other and further features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.